Six infants with anomalous connection of the pulmonary veins to the coronary sinus were studied with single crystal echocardiography. The posterior left atrial wall echo was identified by its continuity with the left ventricular posterior wall echo during a sweep from the left atrium to the left ventricle. In five of the six infants an echo-free space measuring 7 to 9 mm was identified behind the posterior left atrial wall; this space was thought to represent the common pulmonary vein. In four of these five, and also in the remaining infant, a highly mobile linear echo with a double wave form per cardiac cycle was recorded behind the posterior aortic wall and anterior mitral leaflet echoes. Echocardiographic contrast studies after the injection of saline solution into the coronary sinus disclosed that this additional echo was produced by the anterior wall of the coronary sinus. The anatomic connections of the pulmonary veins were established in each patient with pulmonary cineangiography. In four of the six patients all four pulmonary veins were connected to the coronary sinus by way of a common pulmonary vein; in the remaining two patients three of four pulmonary veins were connected to the coronary sinus, while the left upper lobe pulmonary vein was connected to the left innominate vein. The coronary sinus was greatly enlarged in each patient.